Verilog Parameters

Parameters make modules configurable and reusable. They're essential for creating flexible IP that can be adapted to different requirements.

parameter Keyword

Parameters can be overridden during module instantiation:

Parameterized FIFO

module fifo #(
    parameter DATA_WIDTH = 8,
    parameter DEPTH = 16,
    parameter ADDR_WIDTH = $clog2(DEPTH)  // Derived parameter
) (
    input  wire                  clk,
    input  wire                  rst_n,
    input  wire                  wr_en,
    input  wire                  rd_en,
    input  wire [DATA_WIDTH-1:0] wr_data,
    output reg  [DATA_WIDTH-1:0] rd_data,
    output wire                  full,
    output wire                  empty
);
    reg [DATA_WIDTH-1:0] mem [0:DEPTH-1];
    reg [ADDR_WIDTH:0] count;  // Extra bit for full detection
    // ... FIFO logic
endmodule
// Instantiation with parameter override
fifo #(
    .DATA_WIDTH (32),
    .DEPTH      (64)
) u_fifo_32x64 (
    .clk     (clk),
    .rst_n   (rst_n),
    // ...
);

localparam Keyword

localparam cannot be overridden – it's local to the module:

Local Parameters

module state_machine (
    input  wire clk,
    input  wire rst_n,
    input  wire start,
    output reg  done
);
    // State encoding - should NOT be overridable
    localparam IDLE  = 2'b00;
    localparam RUN   = 2'b01;
    localparam WAIT  = 2'b10;
    localparam DONE  = 2'b11;
    reg [1:0] state, next_state;
    // ... FSM logic
endmodule

When to Use Each

parameter – User-configurable values (width, depth)
localparam – Internal constants (state encoding, derived values)

Parameter Override Methods

Override Styles

// Method 1: Named association (RECOMMENDED)
counter #(
    .WIDTH (16),
    .MAX   (1000)
) u_counter (...);
// Method 2: Positional (NOT recommended - error prone)
counter #(16, 1000) u_counter (...);
// Method 3: defparam (DEPRECATED - avoid!)
counter u_counter (...);
defparam u_counter.WIDTH = 16;
defparam u_counter.MAX = 1000;

Common Interview Questions

What's the difference between parameter and localparam?
parameter can be overridden during instantiation; localparam cannot and is always local to the module.
Can you use parameters in port declarations?
Yes, parameters can size ports and internal signals, making modules truly generic.
What is $clog2?
A system function that returns ceiling of log base 2. Used to calculate address width from memory depth.

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